The various issued patents in the ultrasonic and RF respiration monitor field have been the teaching of phase comparison means. Accordingly, some suitable probe is set to emit sufficient energy at a frequency on or about 40 kHz in the case of ultrasound and about 10 gHz in the case of RF, onto the chest cavity or other body portion of the subject. A similar probe to that which was used for transmission, receives some of the energy reflected back from the subject. Movement of the chest cavity and other body parts caused by respiration and heart beat effect the phase of the received energy relative to the phase of the transmitted energy. Since such movements change the length of the path over which the transmitted and received signal travel, the information of movement is contained in the phase. Finally, a suitable phase detector is used to recover the difference in phase which is indicative of movement.
This method of movement monitoring can be effective in certain application. However, the practical utilities of this phase comparison method have not been solved effectively. We can illustrate with the following expression describing the phenomenon EQU .theta.=4.pi.d/L radians (1)
Where
L=wave length C/OF, in cm. PA1 C=velocity of the signal in the medium, in cm/sec PA1 FO=center frequency of the signal, in Hz
From expression (1) it is evident that the phase angle .theta., is a strong function of the path length d. In particular, it is the round trip for the signal from the transmitter probe to the subject and back to the receiving probe. Distance d, and hence phase angle .theta., will change due to the rhythmic physiological movement such as respiration and heart beat, assuming the occurrence of no other physical movement of the subject. More specifically, assume that the total distance d, traveled by the signal from probe to subject and back is several wave lengths (which it will be in a practical application) and .delta.d is the incremental distance change in d, due to rhythmic physiological activity. Accordingly, if d+.delta.d exhibits a phase lag of any integral multiples of 2.pi. radians or more, the phase detector output will exhibit a stationary phase component .theta.s due to d, and .theta..delta. due to .delta.d. The phase detector output in this region, that is about 0 and 2.pi. radians, will exhibit serious ambiguities due to the multivalued transfer characteristics of the device.
The seriousness of the problem can be appreciated when the total path length d, falls right on 0 or 2.pi.. In this situation a double output is given by the phase detector due to .delta.d. Some systems employing this so called open-loop embodiment compromise the operation by having a caretaker physically make an adjustment in distance d, (i.e., move the apparatus slightly closer or further from the subject) in order to operate on the usable portion of the phase detector.
Other teachers of the art use phase lock means to alleviate .delta..theta. from the dependence of the initial value of d. In this embodiment the fixed frequency oscillator is replaced with a voltage controlled oscillator whose frequency is now adjusted by an integrator integrating the output of the phase detector. In this arrangement the loop is closed and, .delta..theta., due to d+.delta.d, always approaches zero because of the integrator in the loop system.
While this phase locked means seems a workable solution the fact remains that such a system suffers, among others, most notably from the requirement of having a very low loop band-width. To illustrate this, it is known from clinical observation that in some situation the respiration rate of a person can be as low as 0.3 Hz. In order to track, and not track out, such a low frequency of motion the loop band-width should be at least about 0.03 Hz. For those who are well-versed in the electronic art know that such a low loop band-width over taxes component values in terms of inherent drift and leakage, resulting in overly touchy circuits, loop instability, slow start-up and frequent dangerous falling out of lock condition. In some application circuitry have been added to indicate loop out of lock, to speed up loop lock and to reduce drift. Invariably, these circuits are complex and, above all, they are also a control system with associated problems of their own. As a result, enhancements such as these can lead to further loop confusion and the over all effectiveness does not prove to be an advantage.
The ultrasonic non-contact motion monitor system of the present invention is distinct from the prior art in the following ways: Unlike the prior art, wherein the teaching is to illuminate the moving subject with waves whose wave lengths much longer then the physical dimensional extent of the movement the present invention teaches that the illuminating wave length must be much shorter then the smallest physical dimens extent of the moving subject. This teaching directs attention to requiring that the wave length of the operating ultrasonic wave be at least one half of the minimum extent of the movement being monitored. This implies that the operating frequency for this invention is to be much higher then previous systems for this application, resulting in dramatic improvement in system performance.
The attention is directed so as to obtain sufficient number of samples from the physical dimensional extent of the movement in order to characterize sufficiently the observed motion. As an example, an assumption can be made that the extent of a one way movement of a subject toward the system is 0.1 cm. In order to get at least two samples, the wave length of the ultrasonic wave should be less then 0.05 cm. Now if the speed of sound in air is close to 34359 cm/sec then the required frequency for the internal system oscillator should be about 687,180 Hz, calculated from the expression FO=C/L. This is nearly 20 times increase over the 40 kHz systems.
Another requirement of the present invention is that the phase of the returned signal from the moving subject is to be compared with the transmitted signal, in-phase, and in phase-quadrature (i.e., shifted by 90.degree.). This is important for identifying movements toward or away from the apparatus. Since this method identifies the direction of motion, a means can be provided to count the chamber of times, in a given interval, change in direction occurred. From this data movement rate is determined.
Increasing the frequency of the monitoring apparatus yield the inherent advantage of narrower beam-width and higher gains from the transmit and receive transducers. Narrow beam-width allows the sonic energy to be focused to a desired spot on the subject thus avoiding the interference from nearby undesirable movements. Additionally, phase locking and the associated problems are avoided; in stead, the system operates on turn-on, requires no critical components and touchy tuning. Furthermore, the initial value of distance d is no longer of any consequence.